Leaked Isil files have revealed the group’s so-called “borders chief”, a senior leader responsible for recruiting thousands of foreign jihadists and funnelling them into Syria.

Tarad Mohamed al-Jaraba, a 37-year-old Iraqi national who uses the nom de guerre Abu Mohamed al-Shimali, facilitated the entry of more than 6,000 fighters over two years.

A third of the 18,000 foreign jihadists who joined the group between 2013 and 2015 put Jaraba down as a “mediator” on their entrance forms, which were leaked to the Syrian opposition newspaper Zaman al-Wasl and shared with the Telegraph.

However, a newly released Isil guidebook shows how the group has adapted by changing how it brings new recruits from Turkey into Syria.

Until recently, all an Isil wannabe had to do to cross into Syria was dress casually and not look religious, the guidebook said.

Isil contacts now meet new recruits in hotels in Istanbul and travel with them to the southern Turkish town of Sanliurfa, where they cross into Isil-controlled Tel Abyad. The route is used because it is closer to Raqqa, its de facto capital in Syria.

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“There is no border crossing here, but there may be guards on the long Syrian-Turkish border. You both look around, and if the coast is clear - they run as fast as they can into Syria, and get into a car of a friend and go to Raqqa.

“You may need wire-cutters, but it is easy.”

Zaman al-Wasl newspaper wrote: “al-Shimali has no competition in the field of propaganda, recruiting and trans-boundary coordination. The man is ranked first in the names mentioned in the Isil archive, with no one else in the organisation figuring anywhere close.”